The present invention relates to workpiece high-speed pressurizing method and mechanism by a cylinder with a cushioning mechanism, the method and mechanism being suitable for use as a gun cylinder or the like for spot welding and more concretely to high-speed pressurizing method and mechanism suitable for a case of pressurizing a workpiece at an end of a projecting stroke of a piston rod by using a cylinder with a cushioning mechanism for reducing a speed of the piston rod near the end of the projecting stroke by restricting a flow rate of exhaust from an exhaust-side pressure chamber.
It is required that a movable-side electrode driven by a gun cylinder for spot welding faces a workpiece on a fixed-side electrode through a relatively short working stroke to carry out a plurality of times of spot welding in a minimum amount of time while changing a welding position. There are also required specifications such as reduction of wear of a tip end of a welding rod (chip) and diminishing of a collision sound. These required specifications can be satisfied by using a cylinder having a cushioning mechanism for reducing a speed of a piston rod near an end of a projecting stroke by temporarily encapsulating air pressure in an exhaust-side pressure chamber but, as a result, the speed of the piston rod is reduced, even though it is temporary. Therefore, this speed reduction becomes a detriment to the requirement of shortening of welding time (speedup).
Moreover, it is required in the spot welding that pressurization with a maximum output is carried out finally even though the speed of the piston rod near the end of the projecting stroke is reduced for reducing wear of the welding rod and diminishing the collision sound and the speed reduction of the welding rod by the cushioning mechanism also delays timing of the pressurization. Furthermore, because a position of the workpiece with which the welding rod comes in contact for pressurization is not a predetermined position due to wear of the welding rod and variations in dimensions of the workpiece, it is not appropriate to control the operation by a projecting length of the piston rod.
The above-described problems are found not only in the spot welding gun cylinder but also in a device such as a pressurizing unit of various clamping devices in which a head mounted to a tip end of a piston rod is repeatedly pushed against a workpiece. In this case, there are similar problems.
It is an object of the present invention to solve the above problems basically by adding simple means to a prior-art fluid pressure cylinder for a spot welding gun or the like or a method for driving the cylinder.
It is more concrete object of the invention to provide workpiece high-speed pressurizing method and mechanism by a cylinder with a cushioning mechanism in which an amount of welding time can be minimized while reducing wear of a tip end of a welding rod or the like and diminishing a collision sound by using the cylinder with the cushioning mechanism for reducing a speed of a piston rod near an end of a forward stroke by restricting a flow rate of exhaust from an exhaust-side pressure chamber.
It is another object of the invention to provide workpiece high-speed pressurizing method and mechanism by a cylinder with a cushioning mechanism in which pressurization with a maximum output can be carried out rapidly after the speed of the piston rod is reduced near the end of the forward stroke and the welding rod or the like comes in contact with the workpiece.
It is another object of the invention to provide workpiece high-speed pressurizing method and mechanism by a cylinder with a cushioning mechanism in which pressurization with a maximum output can be carried out rapidly at an appropriate time irrespective of wear of the welding rod and variations of dimensions of the workpiece by estimating a position of the workpiece with which the welding rod or the like comes in contact from a relationship between pressures of the exhaust-side pressure chamber and a pressurizing-side pressure chamber when cushioning by the cylinder with the cushioning mechanism operates.
To achieve the above objects, a high-speed pressurizing method of the invention includes the steps of: moving a piston rod connected to a main piston forward by supplying air pressure into a head-side pressure chamber of the main piston; reducing a speed of the piston rod near an end of a forward stroke of the piston rod by restricting a flow rate of exhaust flowing out of a rod-side pressure chamber by using a throttle; and then pressurizing the workpiece with the piston rod, wherein internal pressure of the rod-side pressure chamber reduces to be lower than internal pressure of the head-side pressure chamber when the cushioning operates, the reduction triggers operation of a rapid exhaust valve, the rapid exhaust valve opens an exhaust flow path connecting the rod-side pressure chamber and atmosphere to directly exhaust compressed air from the rod-side pressure chamber into the atmosphere, and thus reduction of back pressure of the main piston in the rod-side pressure chamber is sped up to pressurize the workpiece at a high speed.
In the above method, if there is a possibility that the internal pressure of the rod-side pressure chamber reduces to be lower than the internal pressure of the head-side pressure chamber at the improper time, it is preferable that operation of the rapid exhaust valve is triggered by reduction of the internal pressure of the rod-side pressure chamber to the lower pressure than the internal pressure of the head-side pressure chamber and reduction of internal pressure of a flow path on a secondary side of the throttle to a certain low percentage of the internal pressure of the head-side pressure chamber.
A high-speed pressurizing mechanism of the invention comprises a cylinder including a main piston driven by air pressure, a piston rod connected to the main piston, a head-side pressure chamber and a rod-side pressure chamber formed on opposite sides of the main piston, and the cushioning mechanism for reducing a speed of the piston rod near an end of a projecting stroke by restricting a flow rate of exhaust flowing out of the rod-side pressure chamber by using a throttle and a rapid exhaust valve which is connected to the cylinder, operation of which is triggered by reduction of internal pressure of the rod-side pressure chamber to lower pressure than internal pressure of the head-side pressure chamber when cushioning operates, and which opens an exhaust flow path connecting the rod-side pressure chamber and atmosphere to thereby directly exhaust compressed air from the rod-side pressure chamber into the atmosphere.
In the above mechanism, if there is a possibility that the internal pressure of the rod-side pressure chamber reduces to be lower than the internal pressure of the head-side pressure chamber at the improper time, it is preferable that a differential pressure-driven valve for detecting reduction of internal pressure of a flow path on a secondary side of the throttle to a certain low percentage of the internal pressure of the head-side pressure chamber is added to the rapid exhaust valve and that operation of the rapid exhaust valve is triggered by reduction of the internal pressure of the rod-side pressure chamber to the lower pressure than the internal pressure of the head-side pressure chamber and reduction of the internal pressure of the flow path on the secondary side of the throttle to the certain low percentage of the internal pressure of the head-side pressure chamber.
In the invention, the rapid exhaust valve includes a diaphragm, two pressure receiving chambers formed on opposite sides of the diaphragm and connected to the head-side pressure chamber and the rod-side pressure chamber, and an open/close valve provided in the exhaust flow path to open and close the exhaust flow path in synchronization with displacement of the diaphragm.
The differential pressure-driven valve has a valve element for connecting the pressure receiving chamber on one side of the diaphragm to the head-side pressure chamber and atmosphere, two pressure receiving faces formed at opposite ends of the valve element and having different pressure receiving areas, and two pressure receiving chambers for applying the internal pressure of the head-side pressure chamber and the internal pressure of the flow path on the secondary side of the throttle to the pressure receiving faces.
According to a concrete embodiment of the invention, the cylinder has a cylinder tube in which the main piston is housed, a head cover mounted to an end of the cylinder tube, a rod cover which is mounted to the other end of the cylinder tube and which the piston rod passes through for sliding, and intermediate stop position setting means for setting an intermediate stop position of the main piston. The intermediate stop position setting means has a stop position setting piston housed for sliding between the main piston and the head cover, a setting rod connected to the setting piston and passing for sliding through the head cover, a stopper provided to the setting rod to define a stop position of the setting piston, and a contact portion which is formed at the head cover and with which the stopper comes in contact. Furthermore a flow path for supplying and discharging compressed air to and from the head-side pressure chamber is provided in the setting rod.
In the invention having the above structure, when the main piston and the piston rod are driven by air pressure, by restricting the flow rate of exhaust from the rod-side pressure chamber near the end of the forward stroke of the piston rod, the internal pressure of the pressure chamber increases over the head-side pressure chamber and the speed of the piston rod is reduced. At this time, because the compressed air is gradually flowing out of the rod-side pressure chamber through the throttle, the internal pressure which has temporarily increased over that of the head-side pressure chamber gradually reduces as the speed of the piston rod reduces and the internal pressure reduces to be lower than the internal pressure of the pressurizing-side pressure chamber around the time when the piston rod stops at the latest.
Because the piston rod is substantially at the end of the stroke and ready to pressurize the workpiece when the piston rod stops, it is effective to rapidly enhance the pressurizing force by the piston rod. Therefore, the internal pressures of the rod-side pressure chamber and the head-side pressure chamber are constantly compared with each other and operation of the rapid exhaust valve is triggered by reduction of the internal pressure of the rod-side pressure chamber to lower pressure than the internal pressure of the head-side pressure chamber to thereby directly release the back pressure of the main piston of the rod-side pressure chamber into the atmosphere without restricting the flow rate. As a result, the internal pressure of the rod-side pressure chamber reduces extremely rapidly. Therefore, it is possible to extremely rapidly enhance the pressurizing force applied to the workpiece as compared with a case in which the compressed air in the rod-side pressure chamber continues to be discharged through the throttle.
However, in an early stage of the forward stroke of the main piston and in a stage in which the compressed air is supplied to the head-side pressure chamber and the compressed air is discharged from the rod-side pressure chamber, there is a possibility that the internal pressure of the rod-side pressure chamber reduces to be lower than the internal pressure of the head-side pressure chamber. Therefore, there is a possibility that the rapid exhaust valve operates at the improper time if the operation of the rapid exhaust valve is simply triggered by reduction of the internal pressure of the rod-side pressure chamber to lower pressure than the internal pressure of the head-side pressure chamber.
In such a case, if the operation of the rapid exhaust valve is triggered by reduction of the internal pressure of the rod-side pressure chamber to the lower pressure than the internal pressure of the head-side pressure chamber and reduction of the internal pressure of the flow path on the secondary side of the throttle to the certain low percentage, e.g., 35% or lower of the internal pressure of the head-side pressure chamber when cushioning operates by restricting the flow rate of exhaust from the rod-side pressure chamber as described above, it is possible to avoid the above-described operation of the rapid exhaust valve at the improper time.
The rapid exhaust valve is not limited to the above-described structure in which the open/close valve is caused to operate by the diaphragm and various pressure responsive valves for comparing two pressures and operating based on a relationship between degrees of the pressures. If there is a possibility that the pressure responsive valve (rapid exhaust valve) operates at the above-described improper time, it is possible to take measures such as comparing the pressure in the flow path on the secondary side of the throttle and the internal pressure of the head-side pressure chamber and introducing the pressure fluid into the rapid exhaust valve through the valve element which operates when a certain pressure ratio between the pressures is obtained.
Furthermore, in the cylinder, it is possible to stop the main piston in the intermediate position by the intermediate stop position setting means. By the setting of the intermediate stop position, the main piston can occupy an operation preparing position where the piston rod faces the workpiece with a relatively short working stroke. Therefore, it is possible to minimize a length of the working stroke in a plurality of times of spot welding or the like to improve efficiency of the operation. When the workpiece is moved to a position facing the piston rod for replacement or the like, the main piston can be moved to a fully returned position at the maximum distance from the workpiece by canceling setting of the intermediate stop position.